The UK based eBookstore Lovereading.co.uk announced that it had integrated the iBookstore. You can now browse for a title and then buy it from iBooks or from a large number of other eBookstores with a single click. A book listing also offers the price of a paper edition, too.
Be careful; there’s no confirmation button. I accidentally bought a copy of The Da Vinci Code (only thing I bought from iBooks).
Lovereading.co.uk would like you to think it’s the first to integrate iBooks but it’s not. The eBook search engine Inkmesh has supported searching the iBookstore for some time now (6 months?). I’m sure there are others. Of course, one downside of Inkmesh is that it focused on the US, not the rest of the world.